The present exemplary embodiments relate to bisymmetrical electric paper and a system therefor. It finds particular application in conjunction with electric paper applications where it is desired to have the ability to print on either side of the electric paper, and will be described with particular reference thereto. However, it is to be appreciated that the present exemplary embodiments are also amenable to other like applications.
By way of background, electric paper can be defined as any electronically-addressable display medium that approximates paper in form and function. To be most versatile, electric paper should be light-weight, thin and flexible, and it should display images indefinitely while consuming little or no power. In addition, electric paper should be reusable. One must be able to erase images and create new ones repeatedly. Preferably, electric paper should display images using reflected light and allow a very wide viewing angle.
One way to make electric paper possible using traditional electronic display technology is to completely remove the driving electronics from an electronic display package and use external addressing electrodes to write and erase images. This approach both reduces the per unit cost of electric paper sheets and enables the use of cheap, flexible plastic films in place of glass plates for packaging. Multiple electric paper sheets can then be addressed by a single set of external driving electronics, much like multiple sheets of pulp paper are printed on by a single printer.
A known sheet and display system, dubbed Gyricon, is disclosed in various patents and articles, such as U.S. Pat. No. 4,126,854 by Sheridon titled “Twisting Ball Display.” The Gyricon display system is comprised of an elastomeric host layer of approximately 300 micrometers thick which is heavily loaded with rotating elements, possibly spheres, tens of micrometers (e.g., 100 micrometers) in diameter that serve as display elements. Each rotating display element has halves of contrasting colors, such as a white half and a black half. Each bichromal rotating element also has an electric dipole moment, nominally orthogonal to the plane that divides the two colored halves. Each bichromal rotating element is contained in its own cavity filled with a dielectric liquid. Upon application of an electric field between electrodes located on opposite surfaces of the host layer, the rotating elements will rotate depending on the polarity of the field, presenting one or the other colored half to an observer.
A Gyricon sheet has many of the requisite characteristics of electric paper, namely, bistable image retention, wide viewing angle, thin and relatively flexible packaging, and high reflectance and resolution. U.S. Pat. No. 5,389,945 issued to Sheridon on Feb. 14, 1995, and titled “Writing System Including Paper-Like Digitally Addressed Media and Addressing Device Therefor,” describes an electric paper printing system that employs independent, external addressing means to put images on the Gyricon sheets. The external addressing means is described as a one-dimensional array of electrodes connected, either directly or by wireless technology, to modulating electronics. As the one-dimensional array is scanned across the sheet, modulating electronics adjust the potential at the individual electrodes, creating electric fields between the electrodes and an equipotential surface. An image is created in the sheet according to the magnitude and polarity of the electric fields.
FIG. 1 shows a representation of a Gyricon sheet comprised of a plurality of bichromal rotating elements, or display elements, cast in a retaining medium, or media plane, 200. It is contained between a first encapsulating layer, or image plane, 202 and a second encapsulating layer 204. The sheet 200 and encapsulating layers 202, 204 are placed in proximity to a supporting back plane 206 that is electrically grounded. The layer 204 and plane 206 comprise a ground plane. An external addressing device 208 connected to a power supply 210 is depicted moving across the sheet in a direction D. Each bichromal sphere, or display element, 220, 226, 232 is contained in its own liquid-filled cavity 221, 227, 233 within the retaining medium 200. An electric field exists directly between the external addressing device 208 and the equipotential surface 206 that causes the local bichromal sphere 226 to rotate.
To improve performance, more recent embodiments of these sheets advantageously incorporate charge-retaining islands thereon. U.S. Pat. No. 6,222,513 B1, issued Apr. 24, 2001 and entitled “Charge Islands for Electric Paper and Applications Thereof” describes electric paper having these features. Turning now to FIG. 2, an exemplary Gyricon sheet of this type is shown. The Gyricon sheet is comprised of the following elements: a sheet 300, a first encapsulating layer 302 patterned with conductive charge-retaining islands 306, and a second encapsulating layer 304 that may or may not be patterned with charge-retaining islands.
Together, the first encapsulating layer 302 and the second encapsulating layer 304 do the following things: indefinitely contain a sheet 300, provide at least one transparent window through which the sheet 300 can be viewed, and provide at least one external surface patterned with charge retaining islands 306 that can be addressed with an external charge transfer device. The first encapsulating layer 302 and second encapsulating layer 304 could take the form of thin plastic sheets that are sealed or fastened around the perimeter of the sheet 300. The second encapsulating layer 304 need not be entirely separate from the first encapsulating layer 302. The second encapsulating layer 304 could simply be an extension of the first encapsulating layer 302, folded over and around the edge of the sheet and then sealed or fastened around the remaining perimeter. The first encapsulating layer 302 and second encapsulating layer 304 could also take the form of a coating, applied by spraying, doctoring, or some other method to hold the contents of the sheet 300.
FIG. 2 also shows a pattern for the charge retaining islands 306 of the outer surface of the first encapsulating layer 302. Charge-retaining islands 306 have square perimeters and are organized in a regular two-dimensional array. Narrow channels 303 of insulating material separate the charge-retaining islands 306. The channels 303 serve to isolate the charge-retaining islands 306, preventing migration of charge laterally across the encapsulating sheet, and should be small with respect to the charge-retaining islands 306, so that the maximum possible area of the display is covered with conductive charge-retaining material.
FIG. 3 simply illustrates a second possible embodiment of a charge-retaining island pattern that utilizes a random array of islands. The top view of the first encapsulating layer 400 shows randomly shaped and oriented charge retaining islands 404 separated by channels 402. The fraction of surface area covered by charge retaining islands 404 must still be relatively large compared to that of the channels 402, but in such a random distribution, both feature sizes must be much smaller than the pixel size of a displayed image.
The charge retaining islands can be created on or in an encapsulating layer by many means with any conductive material. One technique, which has been tested, creates islands of conductive and transparent Indium Tin Oxide (ITO) on a transparent polyester film. The polyester is coated with a very thin layer of ITO, and then channels are etched in the ITO by photolithographic processes well known in the art. The remaining conductive ITO regions act as charge retaining islands, while insulating channels are created by the underlying polyester. Another technique, called Flexography, has also been used to form these island structures on electric paper configurations.
In U.S. application Ser. No. 10/739,614 filed Dec. 18, 2003, entitled “Flexible Electric Paper Display Apparatus,” naming Gregory P. Schmitz, Eric J. Schrader and Michael B. Heaney as inventors, a double-sided electric paper apparatus is disclosed. As illustrated in FIG. 4, this dual-sided electric paper apparatus 600 is illustrated. As shown, a ground plane 612 is shown as having attached to each side thereof a media plane 618. Attached on opposite sides of the media plane are second encapsulating layers 620 which have formed thereon charge retaining islands 622. Also shown are printheads 624 whose operation is well known in the art. Notably, in this embodiment, the ground plane 612 may take a variety of forms that are described in the application, or others. However, it should be noted that the ground plane is flexible to provide the ultimate flexibility of the paper. Moreover, the ground plane in this configuration is preferably non-transparent so that separate images can be viewed on each side. This embodiment allows for improved flexibility over other embodiments of electric paper that do not use a flexible ground plane.
In U.S. application Ser. No. 10/739,839, filed Dec. 18, 2003, entitled “Improved Flexible and Transparent Ground Plane for Electric Paper,” naming Gregory P. Schmitz and Michael B. Heaney as inventors, are illustrated a variety of embodiments of a ground plane that is discontinuous and is implemented together with a conductive ground wiper that touches the discontinuous ground plane. The conductive wiper ensures that the discontinuous ground plane islands opposite the print head are all at uniform ground when the print head is depositing charge on the top surface of the electric paper to form the image. The discontinuous ground plane may be located in a number of locations, as described in that application. With reference to FIG. 5, an electric paper structure incorporating an example discontinuous ground plane is described. As shown, a sheet of electric paper 1100 includes a media plane 1102 having display elements 1104 disposed therein. Electric paper 1100 is also provided with an image plane 1106 having charge retention devices or islands 1108 formed thereon. Notably, the electric paper includes a ground plane assembly 1120 formed by a base layer 1122 and a plurality of grounding bars 1124. It is the discontinuous ground plane assembly 1124 that provides increased flexibility to allow the electric paper to actually be rolled, similar to actual paper.
This type of construction (e.g., the electric paper assemblies of FIGS. 1–3 and 5) is asymmetrical in two ways. First, an indium tin oxide (ITO) layer on the top surface is patterned into unconnected islands, while the ITO layer on the bottom surface is one continuous layer. Second, a mylar layer on the top surface is a different thickness than the mylar on the bottom surface. As a direct consequence of these asymmetries, it is only possible to print on one side of the electric paper, the island side. Another direct consequence of these asymmetries is the flexibility of the electric paper is different for flexing in a concave manner versus a convex manner. Both of these properties make this construction of electric paper less paper-like. The embodiment shown in FIG. 4 can withstand printing on two sides but has other limitations because of the presence of the ground plane.
An alternative technique for providing conductivity to the surface of electric paper structure has been developed and described in a commonly assigned and co-pending patent application bearing U.S. Ser. No. 10/739,809, filed Dec. 18, 2003, entitled “Disordered Percolation Layer for Forming Conductive Islands on Electric Paper,” and naming Gregory P. Schmitz and Michael B. Heaney as inventors. This alternative approach includes providing an electric paper application including a charge retention layer formed from a disordered mixture of conductive and non-conductive particles. In one form, as shown in FIG. 6, the disordered mixture is attached to a non-conductive surface. For example, randomly mixed and pre-measured amounts of non-conductive particles (e.g., 10 micrometer glass spheres) with conductive particles (e.g., 10 micrometer silver-coated glass spheres) are sprinkled onto a flexible plastic film. The film is coated with adhesive to retain the particles. This film is then joined to the rest of the electric paper structure as an image plane, or encapsulating layer. In another form, as shown in FIG. 7, the randomly pre-mixed amounts of conductive and non-conductive particles are sprinkled directly onto a smooth and sticky layer of the media plane of electric paper. This form provides the advantages noted above as well as the additional advantage that any voltage applied to a conductive island will substantially appear at the surface of the media plane with negligible voltage drop.
In either form, if the relative fractions of conductive and non-conductive particles within the disordered mixture are below a percolation threshold (which can be calculated and measured), then randomly located and shaped conductive islands (comprised of one or more of the silver-coated glass spheres) will be formed. This structure of adhered particles replaces the above-noted conductive island layers formed using photolithographic patterning of ITO films deposited on flexible plastic sheets and other techniques.
In U.S. application Ser. No. 10/927,691, filed Aug. 27, 2004, entitled “Disordered Three-Dimensional Percolation Technique for Forming Electric Paper,” naming Michael B. Heaney and Gregory P. Schmitz as inventors, an electric paper structure is illustrated having a single layer, or sheet, of relatively disordered particles that heretofore were separated as uniquely formed layers in electric paper structures. In this regard, the single layer includes display elements, such as bistable pixel structures and conductive particles, both preferably embedded in an insulating matrix of material (e.g., non-conductive particles).The display elements take the exemplary form of microencapsulated bichromal spheres and the conductive particles serve as both conductive island and as a ground plane. The conductive particles form a discontinuous random layer of conductive islands on one side of the sheet, and a continuous electrically conductive percolative network or matrix, or ground plane on the other side of the sheet. This is accomplished by varying the effective percolation threshold across the thickness of the sheet. That is, particle ratios on one side of the sheet are below the percolation threshold (e.g., forming the conductive islands) while the particle ratios on the other side are above the percolation threshold (e.g., forming the ground plane).